The chromosomes of Lolium temulentum are lon- ger and contain on average 50% more nuclear DNA than the chromosomes of L. perenne. In the hybrid, despite the difference in length and DNA content, pairing between the homoeologous chromosomes at pachytene is effective and the chiasma frequency at first metaphase in pollen mother cells is high, about 1.6 per bivalent, comparable to that in the L. perenne parent. Electron microscopic observations from reconstructed nuclei at pachytene show that synapto- nemal complex (SC) formation in 40% of bivalents is "per- fect," complete and continuous from telomere to telomere. In others, SCs extend from telomere to telomere but incor- porate lateral component loops in interstitial chromosome segments. Even in these bivalents, however, pairing is effec- tive in the sense of chiasma formation. The capacity to form "perfect" SCs is achieved by an adjustment of chro- mosome length differences both before and during synapsis. "Perfect" pairing and SC formation is commoner within the larger bivalents of the complement. At zygotene, in con- trast to pachytene, pairing is not confined to homoeologous chromosomes. On the contrary there is "illegitimate" pair- ing between non-homologous chromsomes resulting in mul- tivalent formation. There must, therefore, be a mechanism operative between zygotene and pachytene that corrects and modifies associations in such a way as to restrict the pairing to bivalents comprised of strictly homoeologous chromo- somes. Such a correction bears comparison with that known to apply in allopolyploids. In the hybrid and in the L. per- enne parent also, certain specific nucleolar organisers are inactivated at meiosis.